Topographical memory impairments after unilateral lesions of the anterior thalamus and contralateral inferotemporal cortex.

Monkeys with crossed unilateral excitotoxic lesions of the anterior thalamus and unilateral inferotemporal cortex ablation were severely impaired at learning two tasks which required the integration of information about the appearance of objects and their positions in space. The lesioned monkeys were also impaired at learning a spatial task and a task which required the integration of information about the appearance of objects and the background on which the objects were situated. Monkeys with only one of the unilateral lesions were not impaired and previous work has shown that monkeys with bilateral lesions of the anterior thalamus were not impaired on these tasks. These results indicate that the whole of the inferotemporal cortex-anterior thalamic circuit, which passes via the hippocampus, fornix, mamillary bodies and mamillothalamic tract, is essential for the topographical analysis of information about specific objects in different positions in space. Together with previous work, the results show that a unilateral lesion may affect cognition in the presence of other brain damage when an equivalent bilateral lesion alone does not. The tasks required the slow acquisition of information into long term memory and therefore assessed semantic knowledge although other research has shown impairment on topographical processing within working or episodic memory following lesions of the hippocampal-diencephalic circuit. It is argued that the hippocampal-diencephalic circuit does not have a role in a specific form of memory such as episodic memory but rather is involved in topographical analysis of the environment in perception and across all types of declarative memory.